Nuclear fuel in water reactors is in the form of sintered pellets that are approximately cylindrical in shape, and that are based on uranium oxide enriched in isotope 235 and/or in plutonium. The fuel may also include a neutron poison and/or a small percentage of metals or oxides for causing grains to grow during sintering and for increasing fission gas retention as provided in WO-A-94/29874. The fuel is used in the form of a stack of pellets in a cladding of zirconium-based alloy for making up fuel rods or elements. Such rods are mounted in a skeleton in order to constitute a fuel assembly. In general, the skeleton comprises two nozzles interconnected by tubes which carry grids for holding the rods at the nodes of a regular array. Assemblies of this type are described, for example, in U.S. Pat. Nos. 5,844,958 and 4,804,516, to which reference can be made.
An imperative in the design of pellets and rods is safety. For economic reasons, it is desirable to lengthen, as much as possible, the time fuel rods remain in a reactor and to reduce the cost of manufacturing pellets. An important factor limiting the operating flexibility of a reactor is the increasing risk of the cladding failing due to mechanical stress caused by interaction between the pellets and the cladding, referred to by the abbreviation PCI (for pellet/cladding interaction).
At present, quasi-cylindrical pellets having a height greater than their diameter have been adopted almost universally. By way of example, pellets that are presently in widespread use have a diameter D of about 358.2 millimeters (mm) and a height H of 13.4 mm, which corresponds to an H/D ratio of about 1.6. Relatively large diametral clearance of about 170 micrometers (μm) is provided between the pellets and the cladding in order to reduce PCI, and recesses are provided in the end faces of the pellets, with such end faces also being chamfered.
Proposals have also been made, as provided in Japanese Patent Application No. JP-A-54 07 4985 for nuclear fuel rods in which the pellets and the cladding are dimensioned in such a manner that the pellets have a height/diameter ratio of less than 0.7, and simultaneously the diagonal of the pellet is less than the sum of the diameter plus the radial clearance, which gives rise to a large amount of radial clearance, thereby constituting an obstacle to heat transfer.